Prion diseases are unique, transmissible, neurodegenerative diseases since the infectious agent consists solely of an alternative conformational isoform of the host-encoded prion protein, PrPSc, that replicates without a nucleic acid (Prusiner, 1982; Prusiner, 1998; Safar et al., 2005). Replication is thought to occur by induction of the infectious conformation in the normal prion protein PrPC (Prusiner, 1982). The different stable conformations, or “conformers”, of PrP have pioneered the concept of protein conformational diseases within the neurodegenerative diseases stating that a misfolded or misprocessed protein is causative in the pathogenesis of the disease (Prusiner, 2001; Taylor et al., 2002). While due to the insolubility of many of the misfolded proteins, structural analysis has been difficult, generation of ligands specific for the misfolded proteins has been key to analyze these protein conformations in their cellular environment (Leliveld and Korth, 2007). The notion that soluble alternatively folded conformers or oligomers of proteins rather than insoluble protein deposits are instrumental in the disease processes has focussed efforts to develop conformer- or oligomer-specific ligands. Conformation-specific monoclonal antibodies (mABs) have been raised to PrPSc (Korth et al., 1997; Paramithiotis et al., 2003) or to Aβ oligomers which are major pathogenic conformers in Alzheimer disease (Kayed et al., 2003), enabling detection of single conformers of proteins within a population of proteins. These reagents have become key reagents in detecting presence of these disease-associated conformers in tissues or body fluids as a method of identifying asymptomatic or early stage individuals at risk to developing prion disease (Kuhn et al., 2005; Nazor et al., 2005) or Aβ-oligomer related disease conditions (Lesne et al., 2006; Luibl et al., 2006).
So far, there is no pharmacotherapy of neurodegenerative diseases aimed at intervening with the fundamental biological causes of these diseases. Active or passive immunizaton approaches targeting disease-associated Aβ conformers in the case of Alzheimer disease (Schenk et al., 1999) or shielding the “normal” substrate conformer PrPC in the case of prion diseases (White et al., 2003) have been performed in mouse models of these diseases. Specifically, administration of mABs in preventing disease-associated symptoms in mouse models both of prion disease and Alzheimer disease (Bard et al., 2000; White et al., 2003). While in the case of anti Aβ mABs, these seem to pass the blood brain barrier (BBB) to prevent Aβ aggregation (Bard et al., 2000), anti-PrP antibodies for preventing prion disease has only been successful after peripheral (intraperitoneal) inoculation when they could act on peripheral sites of replication (Heppner et al., 2001; White et al., 2003). Thus, while anti-Aβ mABs seem to easily pass the BBB, anti-PrP mABs don't.